Ardnacrusha power plant is a hydroelectric power station which was originally referred to as The Shannon Scheme. It is located near Ardnacrusha within County Clare approximately 1.5 miles (2.4 km) from the Limerick border. It is Ireland's largest river hydroelectric scheme and is operated on a purpose built canal connected to the River Shannon. The plant includes fish ladders so that returning fish, such as salmon, can climb the river safely past the power station.

The generating plant at Ardnacrusha is composed of three vertical-shaft Francis turbine generators (commissioned in 1929) and one vertical-shaft Kaplan turbine generator (commissioned in 1934) operating under an average head of 28.5 metres. The scheme originally was designed for six turbines, with four turbines fitted. The 85 MW of generating plant in Ardnacrusha was adequate to meet the electricity demand of the entire country in the early years. The full output equates to about 332,000 MWh generated on an annual basis. Ardnacrusha generates at 10.5 kilovolts (kV) but this is transformed to 40 kV for local distribution and to 110 kV for long distance transmission.

Background

The first plan to harness the Shannon's power between Lough Derg and Limerick was published in 1844 by Sir Robert Kane. Inspired by Nicola Tesla's 1896 project at Niagara Falls, "Frazer's Scheme" proposed a head-race canal ending at Doonass, and was sanctioned by the 1901 "Shannon Water and Electric Power Act". This envisaged a seasonal scheme with a back-up steam turbine to generate electricity in the summer, but the overall cost was considered too great and the Act was shelved. In 1902 SF Dick proposed a sharper fall at Doonass. The British Board of Trade appointed a committee in 1918 which approved proposals by Theodore Stevens and published a report in 1922. This envisaged altering upper lake levels to create extra storage of 10,000 million cubic feet, at a cost of £2.6m. At the same time the First Dail saw the possibilities, and Sean Wall, Chairman of Limerick County Council in 1920 and commander of the East Limerick IRA, asked the Council to make another survey of river levels but was killed in a gunfight at Annacarty in May 1921.

In 1924-25 the new Irish Free State's Minister for Industry and Commerce Patrick McGilligan commissioned the engineer Dr. Thomas McLoughlin to submit proposals. Dr McLoughlin had started working for Siemens-Schuckert, a large German engineering firm, in late 1922, and produced a scheme that would cost £5.2m. This caused considerable political controversy as the new state's entire budget in 1925 was £25m, but it was accepted. The Siemens report drew on earlier hydrological work of John Chaloner-Smith an engineer with the Commissioners of Public Works.

The Shannon Scheme was officially opened at Parteen Weir on 22 July 1929. One of the largest engineering projects of its day, it was successfully executed by Siemens to harness the Shannon River. It subsequently served as a model for large-scale electrification projects worldwide. Operated by the Electricity Supply Board of Ireland, it had an immediate impact on the social, economic and industrial development of Ireland and continues to supply significant power in the 21st century.

In 2002 on the 75th anniversary of the plant, its historic status was recognised by the Institute of Electrical and Electronic Engineers, in partnership with the American Society of Civil Engineers, who marked the facility as an Engineering Milestone of the 20th century.

The Shannon Scheme

In 1925 the works commenced after the passing of the Shannon Electricity Act, 1925 and by 1927 the project was at an advanced state. A completion time limit of three and a half years, with penalty clauses for failure of adherence to this limit, was written into the contract. The final cost overrun was £150,000.

In 1927, the ESB was established and took control of the scheme and electricity supply and generation generally. At the time, it was the largest hydroelectric station in the world, though this was soon superseded by the Hoover Dam, which commenced construction in 1930.

Developed in conjunction with German engineering giant Siemens, most of the skilled workers and engineers on the power station were Germans. A camp was set up for the workers that included living quarters for 750 men and a dining room that seated 600. Initially employment for 700 was provided, whilst at its peak there were 5,200 employed during the construction phase, with this dropping back to 2,500 near completion. The construction project was not without controversy, with national and governmental debate over wages, conditions, strikes, and spending over-runs.

The influential London Financial Times was highly impressed with the result, commenting:

Environmental consequences

The opening of the scheme had, and continues to have, a significant environmental effect on the part of the Shannon bypassed by the head-race canal, from Parteen Villa north of O'Briens Bridge to about a mile north of Limerick City. This length of river, especially that running past Castleconnell and the Falls of Doonass was in the nineteenth and early twentieth centuries world famous for fishing, particularly Salmon fishing. The diverting of water to the power station had a disastrous effect on this, for two main reasons: Initially, there was no fish pass at Ardnacrusha to allow the Salmon to migrate further up the river; this was later rectified. Secondly, the reduction in water flow down the natural channel encouraged more fish to either migrate towards the head-race canal, or to the Mulkear river instead. The problem continues to this day, and the Salmon fishing is now incomparable with the period up to the 1920s. Other species of native fish have also been affected by the low water levels.

Effects on the bypassed river channel

Once opened, the great majority of the Shannon's water was diverted via the head-race canal to the power station. The ESB are required by law to allow 10 cubic meters per second (10 m3/s) to flow down the natural channel. This is roughly what the natural flow would be during dry summer periods prior to the weir being built. All surplus water can be diverted for power generation. The maximum capacity of Ardnacrusha is approximately 400 m3/s, 40 times that which is required to flow down the natural channel (although the power station does not necessarily run at this capacity at all times). For the first few years after the opening of the scheme, water was diverted to the power station only as necessary for the electricity demand at the time, and thus the impact on the river was not initially severe. However, as demand increased, more and more water was diverted, until eventually a situation was reached where, at all times, all available water was diverted for power generation, and the natural channel was permanently reduced to the minimum water flow allowed (except during extreme conditions). In exceptionally wet periods, the flow of water out of Lough Derg is greater than 400 m3/s, and it is then necessary for the surplus to be are released down the natural channel through Castleconnell. During these brief periods, the Falls of Doonass are temporarily restored to their former glory. How often this occurs depends on seasonal weather patterns: some years there is no increase above the minimum flow at all. This has led to a substantially dried-up riverbed. The most obvious result on the river south of Parteen Villa always being kept at summer levels is the silting of many of the old Salmon pools, and the growth of trees and bushes in many parts of the former riverbed, thus significantly altering both the appearance and ecosystem of the river.

When built, Ardnacrusha had the capacity to supply power for the entire country. Currently, it accounts for around 2-3% of the ESB's overall power output,. Given the small overall amount of power produced per cubic meter, there is a substantial case for increasing water flow to the natural channel, now that Arnacrusha is producing so small a proportion of ESB's power. For example, increasing the flow of the river to 50 m3/s would reduce Ardnacrusha's capacity by 1/10 (flow reduced by 40m3/sec), or 8 MegaWatts; less than 0.3% of ESB's national capacity, whilst increasing the water flow to the natural channel 5-fold. This would have a major beneficial effect on the condition of the river south of O'Briens Bridge.

The navigable section of water from the southern end of the Killaloe Canal to World's End, Castleconnell, was linked to Limerick via the Errina to Plassey canal. This became redundant with the construction of the new canal to Ardnacrusha, and subsequently became derelict. Recently, several sections have been cleared, and it is now possible to walk from O'Briens Bridge to Errina lock along the old tow path. As there is no lock at Parteen weir linking the natural channel to Lough Derg, it is no longer possible for any watercraft to enter, by water, this part of the Shannon.

Effects upriver from the scheme

The maximum capacity of Ardnacrusha is approximately 400 m3/s. As this is much greater than is available during summer months, water is stored in the major lakes on the Shannon, Lough Derg, Lough Ree and Lough Allen. By holding these lakes at a higher than natural level, by means of weirs, water accumulated during the wet winter months is released during drier periods to maintain supply to the power station. Weirs already existed at Killaloe and Athlone to control lake levels in Lough Derg and Lough Ree respectively. Upon completion of Ardnacrusha, the weir at Athlone was modified and brought under ESB control, and a new weir built at the mouth of Lough Allen to further regulate water levels (the weir at Killaloe was removed, as Lough Derg's water level is now controlled by Parteen weir itself). As all three lakes now often have a higher water level to supply the power station, this has had the effect of exacerbating the winter floods on much of the Shannon during exceptionally wet conditions.

The scheme simplified navigation between Killaloe and Limerick, as watercraft need to traverse just one double lock at Ardnacrusha. The majority of the Killaloe canal was submerged under the new lake (the 'flooded section') south of Killaloe, allowing direct access to the head-race canal. The ESB is responsible for maintaining water levels for navigation throughout the Shannon to between predetermined limits, but they have the right to prioritize levels for electricity generation, should water shortages arise.